dt-bindings: serial: Add renesas,em-uart bindings

Document Device Tree bindings for the Renesas EMMA Mobile UART
Interface.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/serial/renesas,em-uart.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/serial/renesas,em-uart.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Renesas EMMA Mobile UART Interface
+
+maintainers:
+  - Magnus Damm <magnus.damm@gmail.com>
+
+allOf:
+  - $ref: serial.yaml#
+
+properties:
+  compatible:
+    const: renesas,em-uart
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    const: sclk
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    uart0: serial@e1020000 {
+            compatible = "renesas,em-uart";
+            reg = <0xe1020000 0x38>;
+            interrupts = <GIC_SPI 8 IRQ_TYPE_LEVEL_HIGH>;
+            clocks = <&usia_u0_sclk>;
+            clock-names = "sclk";
+    };